Future digital television (DTV) receivers are expected to be implemented substantially in accordance with the transmission standards established by the Advanced Television Standards Committee (ATSC). A similar standard is the European Digital Video Broadcasting (DVB) standard. A compressed digital video system is described in the ATSC digital television standard document A/53, incorporated herein by reference. Moreover, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1, refers to ISO/IEC standards 11172 and is incorporated herein by reference. The second, known as MPEG-2, refers to ISO/IEC standards 13818 and is incorporated herein by reference.
The new DTV standards allow broadcasters to deliver virtually any format up to 1920×1080 pixels. Specifically, DTV receivers must be capable of receiving source video comprising image sequences that vary in spatial resolution (480 lines, 720 lines, or 1080 lines), in temporal resolution (60 fps, 30 fps, or 24 fps), and in scanning format (2:1 interlaced or progressive scan). Thus, the new DTV standards support either high definition television (“HDTV”), wherein the video frames are of higher resolution than those used in present NTSC signals, or standard definition television (“SDTV”), e.g., television which has approximately the same resolution per frame as the existing analog NTSC standard.
Because of the relatively large amount of data required to represent each frame of a HDTV picture, HDTV decoders must support much higher data rates than SDTV decoders. The additional memory required by a HDTV decoder, as compared to a standard SDTV decoder, and the increased complexity of various circuitry within a HDTV decoder can make a HDTV decoder considerably more expensive than an SDTV decoder.
One prior art technique for reducing memory requirements in a HDTV decoder is identically disclosed in a commonly assigned set of three U.S. patents, namely: U.S. Pat. No. 5,614,952, entitled DIGITAL VIDEO DECODER FOR DECODING DIGITAL HIGH DEFINITION AND/OR DIGITAL STANDARD DEFINITION TELEVISION SIGNALS, issued Mar. 25, 1997; U.S. Pat. No. 5,635,985, entitled LOW COST JOINT HD/SD TELEVISION DECODER METHODS AND APPARATUS, issued Jun. 3, 1987; and U.S. Pat. No. 5,614,957, entitled DIGITAL PICTURE-IN-PICTURE DECODER, issued Mar. 25, 1997. The above three patents [hereinafter the Boyce patents] are herein incorporated by reference in their entirety.
The Boyce patents disclose a series of techniques for reducing the amount of memory required to decode a bitstream including variable length encoded video data. Essentially, high resolution images are decoded at a reduced resolution, thereby requiring less memory to store the images. Moreover, Huffman codes which represent higher-order DCT coefficients are removed form the video stream. Unfortunately, while the Boyce techniques do reduce the total memory and processing requirements of a decoder, the resulting decoder is still quite complex and costly.
Parts of an image to be displayed on a display device, i.e., those regions near the edges of the image, commonly contain information that is not required for enjoyment of the image sequence. This is because of the uncertainty that these portions near the edges will be displayed at all. Such uncertainty is caused by two practices used by many television manufacturers. First, manufacturers may include some amount of “overscan” in their display devices. Second, manufacturers may obscure portions of the picture area near the edges by the use of a display bezel. In recognition of these (and other) practices, the Society of Motion Picture Television Engineers (SMPTE) has adopted a Recommended Practice (RP) 56-1990 which defines a “safe-action” area as the center-most 90% of the image, and a “safe-title” area as the center-most 80% of the image. These portions are linearly determined (i.e., 90% and 80% of vertical and horizontal dimensions).
In view of the above-described SMPTE standard and the need to dramatically reduce system costs in DTV receivers (especially those associated with, e.g., a small display screen), it is seen to be desirable to reduce the video information within an encoded bitstream to correspond to the SMPTE “safe-title” or “safe-action” display sizes.